A quantum well designed to detect infrared (IR) light is called a quantum well infrared photodetector (QWIP). QWIPs operate by photo-excitation of electrons between a ground state and an excited state of its quantum wells. In more detail, a quantum well absorbs IR photons. This absorption of IR photons photo-excite electrons from the ground state to the excited state of each quantum well. The excited states of the quantum wells making up a QWIP are close to or within an energy transport band (sometimes referred to as the continuum or a miniband). A voltage externally applied to the QWIP operates to sweep out the photo-excited electrons from the quantum wells, thereby producing a photocurrent in the continuum.
Quantum wells are grown in a crystal structure. In general, layers of two different, high-bandgap semiconductor materials are alternately grown. The bandgap discontinuity of the two semiconducting materials creates quantized sub-bands in the wells associated with conduction bands. Only photons having energies corresponding to the energy separation between the ground and excited states are absorbed. This is why a QWIP has such a sharply defined absorption spectrum. Note that each well can be shaped to detect a particular wavelength, and so that it holds the ground state near the well bottom, and the excited state near the well top.
FIG. 1 illustrates the conduction-band of a typical QWIP structure and its field distribution. As can be seen, the structure includes a GaAs emitter contact, a stack of quantum well and barrier layers, and a GaAs collector contact. A voltage is applied across the structure, with a ground state near the well bottoms, and an excited state near the well tops. In the stack area, electrons are generated because of light absorption by the wells. Electrons are also present at the emitter area, because it is a doped contact layer. However, there is no photo absorption at the emitter, as there are no quantum wells there. Thus, the only electrons that come from the emitter into the stack are dark electrons, often referred to as dark current.
Because there is no photocurrent generation to cause the electrons to flow from the emitter into the stack, other mechanisms must be used. In particular, the emitter electrons are provided to the stack by one of thermionic emission, tunneling, and thermally-assisted tunneling. Ideally, the field distribution across the device is uniform, as indicated by the constant slope from the emitter to the collector. In actuality, however, the current generated by thermionic emission, tunneling, or thermally-assisted tunneling is substantially smaller than the photocurrent generated by the wells. As such, the actual field distribution is not constant, but effectively adjusts itself to satisfy the requirements for a constant current flow through the device.
In more detail, the actual field distribution (shown as dashed line in FIG. 1) includes a high resistance junction between the emitter and the stack, where most of the device voltage is distributed, as indicated by the dashed line having the steeper slope. This high resistance point is sometimes referred to as the high field domain. The remainder of the voltage is distributed across the relatively lower resistance stack of the QWIP, as indicated by the dashed line having the less steep slope. In accordance with Ohms' Law, therefore, the current flow through the device remains constant, but only at the cost of a non-uniform field distribution.
One adverse consequence of the non-uniform field distribution is the dielectric relaxation effect, particularly in low temperature and low background conditions. More specifically, as a photo-excited electron is swept from a quantum well, it leaves a hole behind referred to as a space charge buildup. In low-background conditions, the high resistance of the high field domain causes delay in refilling the space charge buildup. In this sense, the resistive high field domain and the space charge build up operate to form an RC time constant. This RC time constant results in slow response times. For example, conventional QWIP designs having a large RC time-constant are associated with slow image glow build-up as well as slow image glow fade-out (or image persistence). In addition, such slow response times may render narrow light pulses undetectable by the QWIP.
One QWIP structure designed to overcome this problem separates the quantum wells in the structure using thin barriers, which creates a miniband due to a large overlap of subband wavefunctions. The holes or space charge buildup of depleted wells are quickly refilled with electrons by way of sequential resonant tunneling from the emitter contact layer. In addition, and similar to block impurity band detectors, a thick impurity free blocking barrier is placed between the active well region and the collector contact layer to suppress the dark current of the device. Although such a configuration solves the dielectric relaxation problem, peak sensitivity is weaker and the spectral response is significantly broadened. Such results can be highly undesirable.
What is needed, therefore, is a QWIP design that effectively eliminates or otherwise reduces the dielectric relaxation effect.